Motor having a rotor with interior split-permanent-magnet

ABSTRACT

A motor includes a rotor with interior permanent magnets and a stator with teeth wound by concentrated windings. The permanent magnet is split along a plane oriented to the stator, and an electrically insulating section is set between the spilt magnet pieces. This structure allows the permanent magnet to be electrically split thereby restraining the production of eddy current. As a result, heat-production is damped thereby preventing heat demagnetization of the permanent magnet.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a motor having a rotorwith interior permanent magnets, more particularly it relates to a motorwith interior split-permanent-magnets, thereby restrains eddy-currentfrom occurring and prevents heat-demagnetization.

BACKGROUND OF THE INVENTION

[0002]FIG. 11 illustrates a rotor with interior permanent magnets of aconventional motor. The motor has rotor 310 in which permanent magnets312 are embedded, and rotor 310 is disposed in a stator (not shown) withconcentrated windings, so that the motor can driven by not only magnettorque but also reluctance torque. This rotor is hereinafter referred toas a “rotor with interior permanent magnets”.

[0003] However this conventional motor has the following problems:

[0004] Comparing with a motor with a distributed-winding stator, a motorwith a concentrated-winding stator subjects itself to greater changes ofmagnetic flux interlinked with rotor 310 when the motor rotates. As aresult, a large eddy-current occurs in magnets 312 embedded in therotor, and thus the motor with a concentrated-winding stator isvulnerable to irreversible heat demagnetization. Meanwhile, thedistributed-winding stator is structured in the following way: A slot isformed between two stator-teeth, and a plurality of teeth thus form aplurality of slots. Windings striding over at least one slot areprovided, and part of a winding of a phase exists between pitches ofanother phase winding. The concentrated-winding stator, on the otherhand, is structured by providing a winding of one phase to one statortooth respectively.

[0005] The reason why the motor having the concentrated-winding statoris vulnerable to heat-demagnetization is detailed hereinafter.

[0006] It is well known that eddy current loss “W_(e)” is proportionateto a square of maximum operable magnetic-flux-density “B_(m)”, and thisrelation can be expressed in the following equation.

W _(e) =P _(i)/t={1/(6ρ)}π² f ² B _(m) ² t ² [W/m ³]

[0007] where

[0008] P_(t)=power consumption

[0009] t=plate width interlinking with the magnetic flux

[0010] ρ=resisting value proper to the permanent magnet

[0011] f=exciting frequency

[0012] Since the motor having the concentrated-winding stator issubjected to greater changes in magnetic flux running through the rotor,the maximum operable magnetic-flux-density “B_(m)” in the above equationbecomes greater and thus eddy-current-loss “W_(e)” grows larger.

[0013] If a motor has the concentrated winding stator, and yet, thepermanent magnets are stuck onto an outer wall of the rotor, the changesin magnetic-flux-density is not so large that the heat-demagnetizationdue to the eddy-current-loss is negligible. In the motor having theconcentrated winding stator and a rotor in which the permanent magnetsare embedded, the space between the magnet and the outer circumferenceof rotor core 314 forms a path for the magnetic-flux to flow. Thedensity of magnetic-flux from the stator changes depending on theposition of stator teeth with regard to the magnets, so that magnitudeof changes in the magnetic-flux-density at the path is increased. As aresult, eddy-current occurs in magnets 312 embedded in rotor 310,thereby heating the magnet to produce irreversible heat-demagnetization.

SUMMARY OF THE INVENTION

[0014] The present invention addresses the problems discussed above andaims to provide a motor having a rotor with interior-permanent-magnets.This rotor produces the less eddy-current and can prevent theheat-demagnetization in the permanent magnets embedded in the rotor.

[0015] The motor of the present invention comprises the followingelements:

[0016] a rotor in which permanent magnets are embedded, and

[0017] a stator of which teeth wound by windings in a concentratedmanner.

[0018] The permanent magnets are split in respective sides facing thestator, and insulating sections are inserted into respective gapsbetween respective split magnets. This structure splits the magnetselectrically thereby restraining the eddy-current from occurring andthen suppressing the heat-demagnetization in the magnets embedded intothe rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a cross section illustrating a motor, having a rotorwith interior permanent magnets, in accordance with a first exemplaryembodiment of the present invention.

[0020]FIG. 2 is a perspective view of the permanent magnets to beembedded into the rotor of the motor shown in FIG. 1.

[0021]FIG. 3 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a second exemplary embodimentof the present invention.

[0022]FIG. 4 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a third exemplary embodimentof the present invention.

[0023]FIG. 5 is a cross section illustrating a rotor of a motor, inwhich “I” shaped permanent magnets are embedded, in accordance with afourth exemplary embodiment of the present invention.

[0024]FIG. 6 is a cross section illustrating a rotor of a motor, inwhich permanent magnets are embedded, in accordance with a fifthexemplary embodiment.

[0025]FIG. 7A is a perspective view of permanent magnets to be embeddedinto the rotor of the motor in accordance with the fifth exemplaryembodiment.

[0026]FIG. 7B is a front view of the permanent magnets shown in FIG. 7A.

[0027]FIG. 8A is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a sixth exemplary embodiment.

[0028]FIG. 8B is a front view of the permanent magnets shown in FIG. 8A.

[0029]FIG. 9 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a seventh exemplaryembodiment.

[0030]FIG. 10 is a block diagram of an electric vehicle in which themotor of the present invention is mounted.

[0031]FIG. 11 is a cross section illustrating a conventional motorhaving a rotor with interior permanent magnets.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Exemplary embodiments of the present invention are demonstratedhereinafter with reference to the accompanying drawings.

[0033] (Exemplary Embodiment 1)

[0034]FIG. 1 is a cross section illustrating a motor, having a rotorwith interior permanent magnets, in accordance with the first exemplaryembodiment of the present invention, and FIG. 2 is a perspective view ofthe permanent magnets to be embedded into the rotor of the sameembodiment.

[0035] In FIG. 1, motor 10 includes rotor 14 with interior permanentmagnets 12, and stator 15 facing to rotor 14 via annular space.Respective teeth 17 of stator 15 are wound by windings 18 in aconcentrated manner, i.e. concentrated windings are provided torespective teeth.

[0036] Rotor 14 comprises the following elements:

[0037] a rotor core laminated with a plurality of steel plates;

[0038] permanent magnets 12 embedded into slots axially provided; and

[0039] a rotating shaft 16 extending through a center of the rotor core.

[0040] Respective magnets 12 have a shape protruding toward the centerof rotor core. As such, the magnets are embedded into the rotor so thatrotor 4 can produce respective directions for magnetic flux to flow withease and with difficulty. An inductance ratio in respective directionscan be thus obtained, and it is called a salient pole rate.

[0041] A rotor polarity is formed between magnet 12 and an outer wall ofthe rotor core to which magnets 12 face. The magnetic-flux from thepermanent magnet flows with ease through the section covering the rotorpolarity, and this flowing direction is called “d axis”. On the otherhand, the magnetic-flux flows with difficulty through a section coveringa boundary between two adjacent magnets, and this flowing direction iscalled “q axis”.

[0042] Stator 15 is formed by linking twelve stator-blocks 19 to eachother in an annular shape. Each stator block 19 comprises teeth 17 woundby winding 18 in the concentrated manner, and the blocks are welded toform a ring. In the case of a three-phase and eight-pole motor, forinstance, windings provided to a first four teeth every three teeth outof 12 teeth are coupled with each other thereby forming phase “U”. Inthe same manner, the windings provided to the second four teeth on theright side of the respective first four teeth discussed above arecoupled with each other thereby forming phase “V”. Further, the windingsprovided to the third four teeth on the left side of the first fourteeth are coupled with each other thereby forming phase “W”. Stator 15thus forms three-phase with concentrated windings.

[0043] In motor 1 constructed above, the magnetic flux generated bymagnet 12, i.e. the magnetic flux produced by the rotor-magnetic-poles,travels to teeth 17 of the stator via the annular space therebycontributing to the torque production. This motor has thesalient-pole-rate and controls the current-phases to be optimal bycurrent thereby driving itself not only by the magnet torque but also bythe reluctance torque.

[0044] One of the features of the present invention is a method ofembedding the permanent magnets into the rotor. Magnets 12 to beembedded into rotor 14 in the first exemplary embodiment are detailedhereinafter.

[0045] As shown in FIG. 2, each magnet 12 is split into two magnetpieces 13 in the axial direction of rotor 14. Each two magnet pieces 13are embedded into one single hole provided to rotor 14, thereby formingeach magnet 12. Epoxy resin of electrically insulating, used as acoating material, is applied to the overall surface of each magnet piece13. If magnet pieces 13 are stacked-up, each piece is electricallyinsulated and they can form an independent circuit. A space betweenrespective stacked-up magnet pieces 13 is not less than 0.03 mmcorresponding to the thickness of coating material applied to the magnetpieces.

[0046] The two magnet pieces 13 are embedded adjacently with each otherinto the hole of the rotor core so that magnet 12 is split into twosections facing to stator 15. Respective magnet pieces 13 are arrangedin the following way: Respective magnetic-fluxes generated from twomagnet pieces embedded in one hole flow in the same direction withregard to the outer wall of the rotor to which these two magnet piecesface. Another pair of magnet pieces embedded into a hole adjacent to thehole discussed above generate the magnetic flux in the directionreversed to the direction of the magnetic flux discussed above. Forinstance, two magnetic pieces embedded into one hole face to the outerwall of the rotor with poles “N”, then another pair of magnet piecesembedded into the hole adjacent to this hole should face to the outerwall with poles “S”.

[0047] The space between the two magnet pieces is not necessarily resin,and it can be any electrically-insulating-materials including air-gap.

[0048] Magnet 12 is split by a plane facing toward stator 15, therebyreducing the eddy current produced in magnet 12. The plane extends fromthe rotor center toward the stator. This is because of the followingreason:

[0049] Since teeth 17 are wound by concentrated windings 18, stator 15receives greater changes in the density of magnetic-flux supplied fromteeth 17. The maximum operable magnetic-flux-density B_(m) expressed inthe equation discussed previously thus grows greater. This change in themagnetic-flux-density produces the eddy current in each magnet 12. Inthis first exemplary embodiment, each magnet 12 embedded in rotor 14 issplit into two magnet pieces 13, and epoxy resin—which is non-magneticmaterial—is put between these two pieces, thereby dividing magnet 12 notonly physically but also electrically. As a result, the production ofeddy current is restrained by narrowing the width “t” of plateinterlinking with the magnetic flux in the equation discussedpreviously.

[0050] (Exemplary Embodiment 2)

[0051]FIG. 3 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the second exemplaryembodiment of the present invention. This second embodiment differs fromthe first one in the way of splitting the magnet, and others stay thesame.

[0052] In the first embodiment, the magnet is split into two pieces inthe axial direction, however magnet 22 in this second embodiment issplit into five pieces in the axial direction, and this produces thesame advantage as the first embodiment has done.

[0053] (Exemplary Embodiment 3)

[0054]FIG. 4 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the third exemplaryembodiment of the present invention. This third embodiment differs fromthe first one in the way of splitting the magnet, and others stay thesame.

[0055] In the first embodiment, the magnet is split into two pieces inthe axial direction, however magnet 32 in this third embodiment is splitinto three pieces in a vertical direction with regard to the axialdirection, and this produces the same advantage as the first embodimenthas done.

[0056] The first, second and third embodiments prove that the magnetssplit into pieces along planes facing to the stator can restrain theproduction of eddy current.

[0057] (Exemplary Embodiment 4)

[0058]FIG. 5 is a cross section illustrating a rotor of a motor, inwhich “I” shaped permanent magnets are embedded, in accordance with thefourth exemplary embodiment of the present invention. This fourthembodiment differs from the previous embodiments 1-3 in the shape ofmagnet. In the previous embodiments, the magnet is in a “V” shape;however, magnet 42 in the fourth embodiment is shaped in a letter of“I”.

[0059] In FIG. 5, each magnet 42 formed by two magnet pieces aligned inan “I” shape is inserted into each hole provided in rotor 44.Electrically insulating material is put between the two pieces, thisinsulating material can be air gap. The fourth embodiment can producethe same advantage as the first embodiment has done.

[0060] Regarding the shape of the magnet, the embodiments 1-3 employ “V”shape, and this fourth one employs “I” shape; however, the shape can bean arc being bowed toward the rotor center.

[0061] (Exemplary Embodiment 5)

[0062]FIG. 6 is a cross section illustrating a rotor of a motor, inwhich permanent magnets are embedded, in accordance with the fifthexemplary embodiment. FIG. 7A is a perspective view of the permanentmagnets to be embedded into the rotor of the motor in accordance withthe fifth exemplary embodiment, and FIG. 7B is a front view of thepermanent magnets shown in FIG. 7A.

[0063] In FIG. 6, permanent magnets 52 are embedded in rotor 54, androtary shaft 56 extends through the rotor center. This motor has astator (not shown) disposed around rotor 54 via annular space.

[0064] Magnet 52 is formed by laminating a plurality ofrare-earth-sintered-magnet pieces. Air gaps 58 are provided betweenrespective magnetic pieces. Magnet 52 is bowed toward the rotor center.

[0065] Magnet 52 is further detailed with reference to FIGS. 7A and 7B.

[0066] Magnet 52 comprises rare-earth-sintered magnet. In general, therare-earth-sintered magnet is coated on its surface in order to avoidcorrosion. Magnet 52 is formed by laminating six pieces of thisrare-earth-sintered magnet. Two or more than two protrusions areprovided on the respective faces laminated so that air gaps 58, asinsulating layers, are provided to each magnet piece. The total area ofthe protrusions formed on each magnet piece should be smaller than thearea of the face laminated, e.g. not more than 10% of the facelaminated. The number of magnet pieces is not limited to six but otherplural numbers are acceptable as far as they can provide air gapsbetween each magnet pieces.

[0067] As such, since magnet 52 has insulating layers (air gaps) betweenrespective magnet pieces making up magnet 52, it is difficult forcurrent to run through magnet 52. As a result, the production of eddycurrent is restrained. Meanwhile, magnet 52 employs a conductive coatingmaterial to avoid corrosion; however, the material can be insulatingone, further, respective air gaps can be filled with insulating resinthereby enhancing the strength of magnet 52. The protrusions formed oneach magnet piece can be made from another material and disposed on eachmagnet piece. Electrically insulating material among others for formingthe protrusions can produce the advantage distinctly.

[0068] (Exemplary Embodiment 6)

[0069]FIG. 8A is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the sixth exemplaryembodiment, and FIG. 8B is a front view of the permanent magnets shownin FIG. 8A.

[0070] This sixth embodiment differs from the fifth one in a waysplitting the magnet, and others stay the same.

[0071] In the fifth embodiment, the magnet is split into six pieces inthe axial direction; however, magnet 62 in this sixth embodiment issplit into three pieces in a vertical direction with regard to the axialdirection. The sixth embodiment can produce the same advantage as thefifth one has done.

[0072] (Exemplary embodiment 7)

[0073]FIG. 9 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the seventh exemplaryembodiment of the present invention.

[0074] This sixth embodiment differs from the fifth one in a waysplitting the magnet, and others stay the same.

[0075] In the fifth embodiment, the magnet is split into six pieces inthe axial direction; however, magnet 72 in this seventh embodiment issplit into three pieces in a rotating direction, and a center piece ofthe three pieces is further split into five pieces in the axialdirection. The seventh embodiment can produce the same advantage as thefifth one has done.

[0076] When rare-earth-sintered magnet is used as interior permanentmagnets in the rotor, splitting the magnet effects the advantagedistinctly because the rare-earth-sintered magnet has less electricalresistance and is easier for current to run through comparing with aferrite magnet. (The specific resistance of the ferrite magnet is notless than 10⁻⁴ Ω·m, and that of the rare-earth-sintered magnet is ca.10⁻⁶ Ω·m.) In other words, when the same magnitude of change in themagnetic-flux-density is applied from outside to the magnet, therare-earth-sintered magnet allows the eddy current to run through morethan 100 times in volume than the ferrite magnet does. Thus the split ofsuch magnet effectively restraints the production of eddy current.

[0077] A driving control of the motor is demonstrated hereinafter, thismotor includes the rotor with the interior magnet of the presentinvention.

[0078] A motor with a stator wound by concentrated windings producesgreater changes in the magnetic-flux-density when the motor is drivenunder weakening-magnetic-field control. Because in the motor having arotor with interior permanent magnets, the magnetic-flux runs throughthe space between the magnets and the outer circumference of the rotorcore, and thus the magnetic-flux is distributed unevenly between therotor and the stator.

[0079] The weakening-magnetic-field control applies inversemagnetic-filed to the motor so that the magnetic-flux produced by themagnet can be counteracted, therefore, this control method producesgreater changes in the magnetic-flux than a regular control method does.Further the inverse magnetic-field narrows tolerance for irreversibledemagnetization, and this produces a possibility of heat demagnetizationat a temperature which is a matter of little concern in a normalcondition. The weakening-magnetic-field-control thus produces distinctlyan advantage of damping the heat generated by the eddy current.

[0080] It is preferable to restrain the production of eddy current aswell as the heat-generation from the eddy current by splitting themagnet, and this shows distinctly its effect when the motor is underweakening-magnetic-field-control.

[0081] The motor used in the embodiments discussed above is aninner-rotor type, i.e. a rotor is disposed inside a stator, however, anouter-rotor type, i.e. a rotor is disposed outside a stator, and alinear motor, i.e. a rotor moves linearly with regard to a stator,produce the same advantages.

[0082] As the exemplary embodiments discussed previously prove that themotor with interior permanent magnets of the present invention canrestrain the production of eddy current and damp the heatdemagnetization because the magnet is electrically split and thus anarea of each magnet facing to the stator becomes narrower. The motorunder the weakening-magnetic-field control can further damp the heatdemagnetization.

[0083] (Exemplary Embodiment 8)

[0084]FIG. 10 is a block diagram of an electric vehicle in which themotor of the present invention is mounted.

[0085] Body 80 of the electric vehicle is supported by wheels 81. Thisvehicle employs a front-wheel-drive method, so that motor 83 is directlyconnected to front-wheel-shaft 82. Motor 83 includes a stator beingwound by concentrated windings and having interior permanent magnets asdescribed in the exemplary embodiments previously discussed. Controller84 controls the driving torque of motor 83, and battery 85 powerscontroller 84 and further powers motor 83. Motor 83 is thus driven,which then rotates wheels 81.

[0086] In this eighth embodiment, the motor is employed to drive thewheels of the electric vehicle. The motor can be employed also to drivewheels of an electric locomotive.

What is claimed is:
 1. A motor comprising: a rotor having an interiorpermanent magnet; and a stator having teeth wound by concentratedwindings, wherein the permanent magnet is split along a plane beingoriented toward said stator, and an electrically insulating section isput between split magnet pieces.
 2. The motor as defined in claim 1 ,wherein the permanent magnet is coated by an electrically insulatingmaterial.
 3. The motor as defined in claim 1 , wherein the electricallyinsulating section comprises epoxy resin.
 4. The motor as defined inclaim 1 , wherein the electrically insulating section is formed by airgap.
 5. The motor as defined in claim 1 , wherein the permanent magnetcomprises rare-earth-sintered magnet.
 6. The motor as defined in claim 1is controlled rotation thereof by weakening-magnetic-field controllingmethod.